The present invention relates to a thermal flow sensor in which a heat-sensitive resistor is employed to detect the flow of a fluid.
Hitherto, a method of detecting the flow of a fluid from the equilibrium state of a bridge circuit including a heat-sensitive resistor disposed in the fluid has been applied to thermal flow sensors such as that disclosed in Japanese Utility Model Laid-Open No. 61-108930. A description will be given, with reference to some of the drawings, of a conventional air flow sensor in which a heat-sensitive resistor is employed as a heating resistor which comprises a ceramic substrate and a platinum thin-film resistor formed on the substrate.
FIG. 1 schematically shows the arrangement of a conventional thermal flow sensor in which a heat-sensitive resistor is provided. As shown in the figure, a base 2 is provided at a predetermined position within a housing 1 defining the main passage of a fluid. A heat-sensitive resistor R.sub.H1 and an air temperature sensor Rc are disposed on the base 2. Each of the group consisting of the heat-sensitive resistor RH.sub.1 and a resistor R.sub.1, and the group consisting of the air temperature sensor Rc and another resistor R.sub.2 is connected in series, and these elements form a bridge circuit.
The heat-sensitive resistor R.sub.H1 has a structure such as that shown in FIG. 2. A heat-sensitive resistor portion 7b is formed on one surface of a thin-plate substrate 7a. The thin-plate substrate 7a is disposed in parallel with the direction 6 in which the fluid, e.g., air, flows. This arrangement is provided in order to prevent dust contained in air from depositing on the resistor R.sub.H1, hence, from causing variations in the characteristic of the resistor R.sub.H1.
The thermal flow sensor shown in FIG. 1 also includes a control circuit 10 in which the junction a between the heat-sensitive resistor R.sub.H1 and the air temperature sensor Rc, partially forming the bridge circuit, is connected to the emitter of a transistor 4. Also in this circuit 10, the junction b between the heat-sensitive resistor R.sub.H1 and the resistor R.sub.1, and the junction f between the air temperature sensor Rc and the resistor R.sub.2 are connected to the input terminals of a differential amplifier 3. The output of the differential amplifier 3 is applied to the base of the transistor 4. The collector of the transistor 4 is connected to the positive electrode of a DC power source 5, the negative electrode of the power source 5 being grounded.
The operation of the thermal flow sensor having the above-described construction is already known. Therefore, the operation will not be described in detail, and it will be briefly outlined. When the voltage at the junction b and that at the junction f have become equal to each other, the bridge circuit achieves its equilibrium state. At this time, the heat-sensitive resistor R.sub.H1 allows the passage therethrough of current I.sub.H having a magnitude corresponding to the flow of air. The voltage V.sub.O at the junction b is expressed as V.sub.O =I.sub.H .times.R.sub.1, and the voltage V.sub.O is used as a flow signal.
By virtue of the structure described before with reference to FIG. 2, that is, the structure in which the heat-sensitive resistor R.sub.H1 is disposed in parallel with the air flow direction 6, dust mixed with air forming the flow deposits only on a thick plate portion 11 at an upstream position of the thin-plate substrate 7a of the heat-sensitive resistor R.sub.H1. This permits dust deposition to cause a great extent of variation in the thermal characteristic of the heat-sensitive resistor R.sub.H1.
The conventional thermal air flow sensor having the above-described construction involves the following problem because of the structure where the heat-sensitive resistor R.sub.H1 is disposed in parallel with the air flow direction 6, as shown in FIG. 2. When the heat-sensitive resistor R.sub.H1 is being disposed in the above-described manner so as to serve as the main component part of the air flow sensor, if the angle of the resistor R.sub.H1 relative to the air flow direction 6 deviates from the correct angle, this results in the air receiving area varying to a relatively great extent. In such cases, therefore, there is the risk that a slight deviation in the disposition angle may lead to a considerable level of detection errors.
The problem will be explained with reference to FIGS. 3A, 3B, and 4. FIGS. 3A and 4 are graphs obtained from experiments, and they show examples of variations in detection characteristic from the reference characteristic, which may result from variations in the angle .theta.0 at which the heat-sensitive resistor R.sub.H1 is disposed relative to the air flow direction 6, as shown in FIG. 3B, whereas the reference characteristic is achievable when .theta.=0. FIG. 3A shows shifts in detection characteristic plotted against change in the air flow. FIG. 4 shows the proportion in which detection output varies with variation in the disposition angle .theta. when the flow is a, b or c. It is understood from these graphs that a slight deviation in the angle at which the resistor is disposed results in a great variation in detection characteristic from the reference characteristic that is achievable when .theta.=0. For instance, when the flow is c, if the disposition angle .theta. deviates by 5 degrees, detection characteristic varies by + 13%. When the disposition angle .theta. is a large value, detection characteristic varies to a slight extent. For instance, when the flow is c, if the disposition angle .theta. which is 30 degrees changes by 5 degrees, substantially no change occurs in the detection characteristic.